Most Swahili noun class prefixes are overt, e.g. <i>m-toto/wa-toto </i>‘child/children’. However, for Classes 5 and 9/10 an overt prefix occurs only in certain morphophonemic contexts. Despite superficial similarities, only the Cl. 5 prefix should be analyzed as zero (meaningful absence); Cl. 9/10 nouns simply lack a prefix. Evidence includes differences in singular-plural patterns and in derivational productivity of prefix absence. Prefix absence indicates (inherent or derived) Cl. 5 membership and singular number. The availability of prefixless Cl. 9/10, outside the normal class and number systems, helps preserve the coherence of the noun class system, and allows zero to convey meaning in the case of Cl. 5.